The present invention relates to ceramic dielectric compositions and, in particular, to ceramic dielectric compositions which, when sintered, have dielectric constants between 2400 and 3100, low dissipation factor (DF), e.g., below about 2%, high insulation resistance capacitance (RC) products, e.g. above 5000 ohm--farad at 25.degree. C. and above 1000 ohm-farad at 125.degree. C.; stable temperature coefficients (TC), in which the dielectric constant does not alter from in base value at 25.degree. C. by more than 25% over a temperature range from -55.degree. to 125.degree. C.; a near zero porosity level; and an average particle size of less than 0.8 microns.
The ceramic compositions of the present invention are useful in manufacturing multilayer ceramic capacitors (hereinafter MLC) which require a high capacitance and which typically have a relatively small size. The ultra low porosity of these compositions, combined with the submicron powder particle size, enables thinner layers to be made in the MLC, which results in a higher capacitance and reliability of the device. MLCs are commonly made by casting or otherwise forming insulating layers of a dielectric ceramic powder upon which conducting metal electrode layers, usually consisting of a palladium/silver alloy, are deposited. The ceramic composition must then be fired at temperatures greater than or equal to 1280.degree. C. to form the MLC device.
It is well known that temperature stable ceramic dielectrics may be prepared by mixing pure barium titanate with other minor additives for control of the final dielectric properties. Using commercially available high purity barium titanate produced by chemical coprecipitation by the solid state method, downward Curie shifters can be added to shift the Curie peak of the BaTiO.sub.3 from about 125.degree. C. to room temperature where it is desirable to have a high dielectric constant. The stability of the dielectric constant over a wide range of temperatures, its insulation resistance, reliability, and other factors influence the final ingredients to be used in a dielectric composition.
Typically barium titanate powders are normally physically characterized as having an average particle sizes of greater than 1.0 micrometers in diameter. This can be achieved by either air impact milling or wet milling. As MLC technology is going towards greater miniaturization, it is more desirable to have thinner dielectric layers which will achieve a higher capacitance with the same design. Submicron particle size powders are desirable for thinner and thinner layers. Typical ceramic dielectric compositions which are disclosed in the prior art, such as U.S. Pat. Nos. 4,882,305 and 4,816,430, lose their stability, especially TC characteristics, as the particle size of the dielectric powder is reduced to less than 0.7 micrometers. Also, there is a small population of large size pores apparent in MLCs made with dielectric powders of the standard particle size. Although these pores do not affect the electrical properties and reliability of MLCs having dielectric layers of thickness 25 micrometers, the large size pores become increasingly undesirable as the dielectric layers become thinner and thinner. These large pores of approximately 5 to 7 micrometers in size will eventually cause problems of electrical degradation and reliability.
It is known in the art that when the particle size of barium titanate based compositions is reduced by milling, an improvement of porosity is achieved. However, currently available milling methods have side affects such as a noticeable clockwise rotation of the TC curve. Attempts to counter this effect by varying the ingredients of a standard high fire X7R composition (such as the ratio or level of the Nb.sub.2 0.sub.5 /CoO additives) have been unsuccessful to date. A high fire X7R prior art composition was studied but could not maintain a stable X7R TC, which is that the capacitance does not vary by more than .+-.15% over the temperature range of -55.degree. C. to 125.degree. C. when the particle size was reduced to below 0.8 micrometers. A typical example is given below.
TABLE I ______________________________________ Average particle 1.3 1.02 0.8 0.72 0.58 size (.mu.m) Dielectric Constant 3458 3595 3767 4047 4386 TC (%) @ -55.degree. C. -2.60 -0.8 -6.4 -18.5 -34.9 25.degree. C. 0 0 0 0 0 85.degree. C. -4.4 -6.5 -10.4 -16.6 -23.4 105.degree. C. -3.2 -6.1 -11.3 -19 -29.4 125.degree. C. 7.7 2.3 -6.1 -17.1 -32.1 ______________________________________
Without changing the dielectric composition, the dielectric properties listed above indicate that when the average particle size is reduced to less than 0.8 micrometers the TC can no longer meet the Electronic Industry Association X7R TC specification. It is noted that the data given above in Table 1 are for disc ceramic capacitors. It is well known to those who are familiar with the art that the TC of a ceramic multilayer capacitor is expected to be at least 4% more negative on the high temperature side due to the reaction of the ceramic with the internal electrodes and due to the dielectric thickness. Therefore, based on Table 1 data, powders with a particle size of less than 0.8 micrometers will have a TC lower than -15% when applied in MLCs even though the TC is within -15% in the disc.